This invention relates to measuring devices for measuring properties of specimens, and, more particularly, to an ultrasonic device for measuring such properties.
Structures are built from materials which are normally selected for their high modulus and strength, and for acceptable collateral properties such as resistance to environmental damage, fatigue resistance and fabricability. For many years, metal alloys were the preferred structural materials for aircraft and spacecraft applications. More recently, composite materials made of bonded mixtures of different components have been developed and refined for use in specific applications. Composite materials can be prepared that have higher elastic modulus and strength per unit weight than metals, and therefore have become of great interest for use in advanced structures.
The properties of composite materials can be varied over wide ranges through control of the properties of the components and the amounts of each component present. Composite materials can be intentionally fabricated to have specified elastic and strength properties in the directions wherein such properties are required, and can even be tailored to have different properties within a single continuous part. The composite materials therefore offer designers the opportunity to greatly improve the performance of structures, but also impose some additional burdens on those who build and maintain the structures. That is, since the composite materials are of such a nature that they can be fabricated with widely varying properties, each composite structure that is built must be carefully monitored to ensure that it is within the limits specified by the designers.
One of the currently most important types of composite materials is fiber composites of graphite, glass or Kevlar fibers in a thermoset or thermoplastic matrix, which are used in advanced aircraft and spacecraft structures as well as commercial applications such as tennis rackets and golf clubs. These composite materials are a mixture of oriented or unoriented fibers in a matrix which binds the reinforcement together and also protects it. It is vital to know accurately the fractions of each of the components present in the material and the amount of moisture present in voids within the material, as large variations in component fractions and too much moisture may lead to otherwise undetectable sources of failure.
Parts made from such fiber composites are usually fabricated by filament winding or by bonding together thin plies of "prepreg", a precursor material made of the reinforcement fibers in an uncured matrix that is available as sheets about 0.004-0.008 inches thick. The prepreg is of interest in itself, as its properties must be evaluated during manufacture and prior to bonding to be certain of its quality. Sheets of the prepreg are stacked together or "laid up" to form thick pieces termed laminates, and these laminates are cured in autoclaves to form semifinished parts.
The local weight fraction of the resin matrix and the reinforcement fibers, and the local presence of moisture should be known for both the prepreg and the cured final part. In the latter case, it is desirable that the moisture content of the composite material be known for the prepreg, upon curing, and also after field service. Moisture can be absorbed into these composite materials during fabrication of the prepreg and the layups, or during service. The moisture is highly damaging to the material, and can lead to premature failure of the part.
At the present time, information about the local weight fraction of reinforcement fibers and matrix can be determined readily only by destructive testing, and information about the moisture content can be determined only by laborious and partially destructive techniques such as desiccation. In the usual commercial procedure for determining weight fractions, a piece of the composite material is cut away from the rest and weighed. The matrix is then chemically or thermally removed, leaving only reinforcement particles. The particles are weighed, and the weight is divided by the total weight of the piece to determine weight fraction. From calibration tables or known transformations, the volume fraction is calculated. The weight fraction of matrix is calculated as one minus the weight fraction for the fiber. Where volume is conserved, the volume fraction of matrix is one minus the volume fraction for the fiber. Measurements of moisture are accomplished by vacuum desiccation of pieces of the composite material, weighing the material before and after desiccation to determine the moisture lost. Both types of measurements cannot be done with a part in service without having the evaluation procedure itself do significant harm to the structure, unless test coupons are built into the part. Even then, the actual local composition and moisture content cannot be evaluated, only inferred from measurements of neighboring areas.
Composite materials are entering more widespread use in applications such as commercial and military aircraft structures for which the fractions of the phases and the moisture content, as well as any microstructural irregularities, must be accurately known both at the time of manufacture and during service. Deviations from specified values can lead to local weaknesses, which in turn might result in failure of the part made of the composite material. An accurate, reliable approach to measuring the characteristics of materials formed of mixtures is required so that the prepreg starting material, the laminates, and the service part can be readily sampled and evaluated.
The preceding discussion has focussed on one specific type of mixture, composite materials. However, the problem of determining the component fractions of a previously formed mixture is found in many other areas, including geology, mineralogy, construction, automotive tire production, and manufacturing. A solution of the problem for mixtures such as composite materials would likely be applicable to, and would provide valuable benefits in, these fields also.
Accordingly, there exists a need for a testing device which permits a determination of the weight fractions of the phases, and the moisture content, in a mixture such as a composite material. The apparatus must permit testing of prepreg and finished parts, without damaging the specimens and in a manner that is economical and consistent with large scale testing. The present inventors have determined that such measurements can be accomplished with ultrasonic apparatus, but have found that the existing types of ultrasonic measurement apparatus cannot make the required measurements and meet the above-stated requirements. The present invention fulfills the need for an ultrasonic apparatus meeting these requirements, and further provides related advantages.